Magneto-type ignition systems for an internal combustion engine are well known; the referenced patent, assigned to the assignee of the present application and the disclosure of which is hereby incorporated by reference, U.S. Pat. No. 3,963,015, Haubner et al, describes a magneto ignition system in which a magnet wheel is being rotated by the engine, for example a small internal combustion engine (ICE) of the type used for lawnmowers, chainsaws, and the like. The magnet wheel is magnetically coupled to an ignition magneto armature. As the magnet passes the pole shoes of the armatures, an alternating voltage is induced in windings on the armature. The alternating voltage is formed by a small negative half-wave, a main positive half-wave and a subsequent negative half-wave. The negative half-waves are applied via a rectifier arrangement to the primary current circuit, which includes an electronic ignition switch. The details of such a circuit are explained in the referenced U.S. Pat. No. 3,963,015. In an upper speed range, the smaller first half-wave is used to obtain an advance of the ignition instant, that is, to advance the ignition timing. The advance is not gradual, rising with speed, but, rather, jumping from a predetermined ignition timing to an advanced timing instant. As the speed of the ICE increases, the halfwaves, including the negative half-waves, increase. The speeddependent increasing half-waves control, via a control circuit, an ignition switching element, typically a transistor, when a predetermined primary voltage is reached.
It is frequently desirable to change the ignition timing in dependence on the speed of the ICE gradually, that is, with increasing speed to advance the ignition timing. This is preferred over the jump adjustment. It is known to determine the ignition instant by using a control circuit as described, for example, in German Patent Disclosure Document No. DE-OS 33 25 275. The there disclosed system does not utilize rectifiers to rectify the smaller negative half-waves, since ignition is carried out over the entire speed range of the ICE, and hence of the magneto, with the larger, positive half-waves. Magnetos having a magneto armature of generally E shape have a disadvantage, however, in that the smaller, negative half-waves, which occur in advance of the actual positive ignition half-wave, is considerable when the speed reaches its upper speed range. This negative half-wave which is also induced in the secondary winding may be sufficiently high so that the induction voltages cause sparks to flash over on the spark plug, resulting in far too advanced sparking and misfires.
It has been proposed to eliminate such misfires by damping the negative half-waves in the primary circuit. A suitable diode circuit and a serially connected resistor bridges the primary winding--see, for example, German Patent Disclosure Document No. DE-OS 27 30 002. Such an arrangement requires additional circuit elements, and substantially increases the power losses in the control circuit; additionally, by armature reaction, the beginning of the subsequent ignition half-wave is delayed.